Internal excitation of a gear pair is generated by an inner system during a gear pair meshing process, which is also a difficult and important point in the gear system kinetic study. Internal excitation of a gear system is divided into three manners: time-varying stiffness excitation, time-varying error excitation, and meshing contact excitation. Time-varying stiffness excitation may be associated with three parameters: bending stiffness of gears, shearing stiffness, and contact stiffness. IO56336 meshing stiffness calculation formula and taking gear stiffness excitation equivalent to Fourier expansion are extensively used currently. Hertzian contact theory based on a smooth surface is further used for deducing contact stiffness calculation. However, deformation of a contact state of rough surfaces of actual gear may not be simulated precisely. To build more effective gear kinetic models, there is a need for improvement of devices and methods for measuring three-dimensional contact stiffness of spur gear based on rough surface.